This invention relates to improvements in magnetically actuated sensing devices for monitoring the opening or closing of doors or windows or the like as a part of an electrically monitored physical security system.
Magnetically actuated switches previously used in physical security monitoring systems have typically utilized a magnetically actuated reed switch mounted, for example, in the frame of a doorway or window, with conductors leading therefrom to a security system control unit, and one or more magnets mounted on the edge of the door or window for actuating the reed switch. When the actuating magnet approaches the reed switch within a certain distance, determined by the sensitivity of the reed switch and the strength of the actuating magnet, the magnet actuates the reed switch by closing a set of magnetic contacts therein, which sends an electrical signal to a control unit.
In such applications it is desirable to utilize a relatively sensitive switch so that the distance between the switch and its actuating magnet necessary to actuate the switch is not critical, thereby permitting slight variations in the position of the magnet without changing the state of the switch and setting off a false alarm. A previously known improvement on such magnetically-actuated reed switches for increasing their sensitivity uses a small permanent magnet placed close to a reed switch to bias the response of the switch to the presence of an external magnetic field, so that a weaker magnetic switch field is required to actuate the magnet. Adjustable biasing may be achieved, for example, by variation of the distance or angular relationship between the magnet and the longitudinal axis of the reed of a switch, as shown in Nicholls U.S. Pat. No. 3,974,469, and by varying the location of the reed along an imaginary axis parallel to the axis of polarity of the biasing magnet, as shown by Tann U.S. Pat. No. 3,305,805. An example of a previously known switch using a biasing permanent magnet in proximity to a reed switch is the model MDS-B switch manufactured by the Potter Electric Signal Co. of St. Louis, Mo. In that device a movable actuating magnet is oriented in magnetic opposition to a biasing magnet located near a reed switch such that the biasing magnet improves the sensitivity of the switch.
Another desirable feature of switches used in security systems is resistance to manipulation or deception by the use of foreign magnetic fields. High-security switches known as "balanced" switches have been developed for this purpose. An example of this "balanced" type of switch device is the model DR-850 switch manufactured by Walter Kidde & Co. of Belleville, N.J. which has two single-pole-double-throw reed switches and a biasing magnet associated with one of them. An actuating permanent magnet is provided whose position is adjustable within its housing so that when properly adjusted the reed switches will be operated to produce a "normal" indication. However, any attempt to defeat the system with an externally applied magnet, regardless of its field direction, is alleged to upset the balance of the switches and thereby produce an "abnormal" indication. Such "balanced" high-security switches, however, must be carefully adjusted during their installation to provide proper actuation.
Adjustment of the position of the actuating magnet relative to the switch is critical in the prior art devices adapted for use in security systems, and when seasonal changes in air temperature and humidity cause changes in the alignment of a door to its frame minor misalignment is often sufficient to cause the device to produce false indications, requiring expensive service calls for realignment of the switch and its actuating magnet.
Additionally the prior art devices require further adjustment for use in environments including significant amounts of magnetic material, as when they are mounted on steel doors and doorframes, and covers on such devices that are removable for performing such adjustments therefore also require security devices such as tamper detection circuits. None of the aforedescribed prior art devices solves all of the problems confronting security devices in an entirely satisfactory manner.
Accordingly, a switch is needed which is difficult to deceive using additional magnets, and which remains sensitive to opening of the door or window that it monitors without the necessity for adjustment or access to internal components.